1. Field of the Invention
The present invention relates to crystallite dispersions. More particularly, the invention relates to crystallite dispersions of unsaturated polyesters in a coherent phase of conventional polyols, the process for their manufacture, and their use in polyurethane and polyurethane group-containing polyisocyanurate foams.
2. Description of the Prior Art
Cellular and non-cellular polyurethanes and polyisocyanurates have previously been prepared from dispersions of aromatic polyesters in polyhydroxyl compounds, which are liquid at room temperature. These dispersions may be prepared, for example, by dispersing the polyester melt under high shear gradients as disclosed in European published application No. 17,111. It is characteristic of this method of preparation that the dispersed phase is not bonded into the polymer structure when the polyurethane is prepared, but instead is present in the molded part as a filler which has a reinforcing effect. It is characteristic of this approach that the temperature of the polyurethane polymerization reaction must be chosen such that the melting point of the crystalline polyester is not reached.
Crystallite suspensions are also disclosed in European published application No. 32,380. These crystallite suspensions contain flexible polyols as the coherent phase, while the disperse phase is comprised of rigid, crystalline organic compounds having at least one Zerewitinoff active hydrogen atom. These crystallites possess melting points of from 30.degree. to 260.degree. C., and molecular weights of from 178 to approximately 100,000. This process has the advantage that the rigid, crystalline organic compound may be incorporated into the polyurethane structure in a carefully controlled manner by controlling its melting point or the transition from the dispersed to the coherent phase. As a result, added strength may be given to previously formed cell membranes. One disadvantage of this process is that the crystallite suspensions are very difficult to manufacture in a reproducible fashion.
An improvement in the reproducible preparation of storage-stable crystallite suspensions is described in European published application No. 62,204. When the reaction conditions such as agitator speed and temperature/time profile are carefully controlled, stable crystallite suspensions are obtained whose dispersed phase is comprised of up to 50 weight percent particles whose size ranges from 0.5 to 100 .mu.m. However, random production variations sometimes produce crystallite suspensions in which more than 50 weight percent of the dispersed polyesters have a particle size smaller than 10 .mu.m, so that thickening occurs when the crystallite suspensions are stored due to thixotropic effects. Such crystallite suspensions may no longer be processed into polymers by means of isocyanate addition polymerization. A further disadvantage is that crystallite suspensions prepared according to European published application No. 62,204 tend to form voids, which greatly reduce the quality of polyurethane group-containing polyisocyanurate foams prepared from these dispersions. Finally, these crystallite suspensions also all have the disadvantage that the polyurethane or polyurethane-group-containing polyisocyanurate foams prepared from them generate a relatively high smoke density when burned.
In order to prepare high-density polyurethane or polyurethane-group-containing polyisocyanurate molded parts or thick sandwich elements, the percentage of dispersed phase in the crystallite suspensions must be maximized, and the compressive strength of the molded parts obtained by using crystallite suspensions must be improved. The percentage of conventional reactive and nonreactive flame retardants in the polyurethane- or polyurethane-group-containing polyisocyanurate polymers must be reduced, and the smoke level significantly lowered when the material burns, in order to produce high quality, economical products.